How to set a column value based upon duplicate records

Suppose I have the following data in a table named "Teams"
If I have a table of football teams.
I want to update the table and set the "Flag" column to 1, if there are any duplicate football teams in the table.
The TeamId will determine if a row is a duplicate, since the TeamId would occur more than once. Here is an example of the table before the update.

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